[Update #2]: The latest preview has finally offered a close-up look at the Immortal Empires campaign map and some of the new Legendary Lord positions.
[Update]: Total War: Warhammer 3's first post-launch DLC will add four new Chaos factions.
Once it arrives this August, Total War: Warhammer 3's Immortal Empires campaign map will combine all of the major gameplay elements and factions from all three games into one gigantic package. You can expect to see 86 playable Legendary Lord, dedicated factions for a bunch of mostly forgotten characters like Grombrindal and Volkmar the Grim, new starting locations for 29 of the older Legendary Lords, as well as improved faction mechanics for a fair few of them.
Best of all, Creative Assembly has now revealed that the Immortal Empires combined campaign map will be nearly twice as large as the Mortal Empires map first introduced in 2017. Not only that, but there's a whole bunch of empty space on the new map that's obviously going to be expanded upon in the future, so the final version of Immortal Empires should be remarkably massive.
You can find the full preview over at the Total War: Website, but for now there are two areas I feel are worth discussing: the expanded Southern Realms and the southeastern lands of Khuresh and Ind. The first to me indicates that the mercenary-focused Dogs of War will likely be added as a DLC expansion. Almost certainly not soon as the often-rumored Chaos Dwarfs are probably going to be the first race pack, but I'm sure they'll eventually make their way into the game given their popularity among the community.
There is a lot of space to expand the game in the southeast
As for the second part, Creative Assembly has already stated that neither Khuresh nor Ind are currently in the plans. That said, with their lands now visible on the campaign map and Total War: Warhammer 3's development likely to continue for many years, I wouldn't be surprised if that attitude changed at some point. Still, don't get too excited just yet as in all likelihood we're going to have to wait a few years for things like snake-men.
The final thing worth mentioning are the Sea Lanes that allow you to traverse from the East end of the map to the West and vice versa. Doing so will take you a couple of turns, but it's a price well worth paying since it'll greatly expand the amount and diversity of faction you can fight in a single campaign. So for a small addition, I have a feeling Sea Lanes will be quite an impactful one!
Once Creative Assembly reveals more information I'll make sure to let you know as I'm rather intrigued myself. Until then, consider giving Total War: Warhammer 2's Mortal Empires a try. It's not as big as the upcoming Immortal Empires, but it's still a gigantic world full of interesting new people to meet and then subsequently squabble with!